In propeller drives where an exhaust duct is divided into two duct sections as indicated above, a seal is required between the two duct sections in order to avoid exhaust gas leakage when the boat is driven, at least above a certain minimum speed. This minimum speed may be, for example, 3–5 knots and can also be said to correspond to a practical upper limit for driving the boat in a harbor area or in proximity to another mooring. If exhaust gases are allowed to leak out between the duct sections when the boat is driven above said minimum speed, exhaust gases may be drawn into the boat at the stern where a local negative pressure then prevails. This effect is sometimes called the wagon-back effect. An undesirable exhaust gas discharge between the duct sections when the boat is driven at a speed exceeding said minimum speed also leads to unfavorable hydrodynamic flow conditions arising in the transition region between the fixing plate and the underwater housing, which has a negative effect on the propulsion of the boat.
An obvious and generally well-known way of sealing exhaust ducts which are movable relative to one another is to arrange a sealing flexible exhaust bellows made of rubber or rubber-like material between the duct sections. A problem with such a solution in this case, however, is that the exhaust bellows is relatively bulky in the vertical direction, in particular when it has to cover a certain rotation range for the propeller drive.